A joke circulating on social media suggests how President Trump could arrest terrorism against Indian Kashmir by jihadist groups based in Pakistan—a country that at its worst actively arms and shelters these terrorists, and at its most benign turns a blind eye. Let Mr. Trump open a large resort in Kashmir, the joke goes. Give him rent-free land on a long lease for golf courses and ski lodges—and then watch how quickly the terrorism falls off. Humor aside, Mr. Trump has helped to calm the region and scored his first major diplomatic achievement. On April 22, terrorists said by India...